CFD produced streamline velocities used to identify the flow into the propeller plane

Designing efficient hulls using CFD

21 January 2015 by Richard Broughton

Superyacht buyers are becoming increasingly keen to ensure their new purchases are as efficient as possible – they simply cannot afford not to, as the cost of fuel and meeting various environmental regulations increase. Computational fluid dynamics (CFD) is one method how owners are ensuring their yachts achieving high levels of efficiency.

CFD has been used in the large commercial sector since the 1980s. But its until now its application in the superyacht arena was underutilised, limited to designing underwater appendages rather than designing hull forms.

Sergio Cutolo, founder of naval engineering company Hydro Tec, understands the difference CFD can make but takes a practical approach to its use for underwater appendages.

‘Computational fluid dynamics is still very expensive compared with traditional tank testing, due to the processing power and software required,’ he explains. ‘We use tank testing for hull design, although very often the testing facility is performing some CFD prior to model construction.

‘We then use CFD for the details, such as propeller tunnels, shaft lines, rudders, and interaction between stabilisers and hull.’

Established in 1995, Hydro Tec, based in Varazze, Italy, embraces a wide range of naval architecture projects but has always held the use of computer technology to advance yacht design in high regard.

‘CFD is still very demanding in terms of software and in terms of training,’ says Cuttolo. ‘Our services include hull lines design, but they are limited to this kind of activity. Every year we have to develop five to ten new hull forms and, in my experience, it is not really cost effective to have an in-house team for CFD with the work we are doing.

‘However, being able to utilise external organisations that can perform CFD is key to a deeper, more comprehensive understanding of the flow around the hull of large yachts. It is certainly the way the market is going; it has to.’

CFD produced streamline velocities used to identify the flow into the propeller plane

CFD’s wider application

One such organisation that is developing these resources in-house is CJR Propulsion. The Southampton-based company has been designing and building precision propellers and sterngear for decades and has invested heavily in the latest research and technology, including robotic manufacturing and ‘true 5-axis’ CNC machinery.

This aspiration led CJR’s managing director, Mark Russell, to look to the industry to investigate how CFD could be used to improve performance and efficiency. The culmination of which was the formation of a partnership with Southampton University’s Fluid Structure Interactions Research Group (FSIRG), the UK’s leading academic institution for marine technology.

Working together to develop accurate design tools to optimise propeller and sterngear technology for improved performance, fuel efficiency, longevity and reduced vibration, CJR’s in-house R&D team was joined by FSIRG’s Simon Lewis, a post-doctoral research engineer, who specialises in CFD.

The partnership also allows CJR to utilise the university’s resources, such as Iridis 3, its advanced supercomputer, which would be otherwise unobtainable for CJR.

‘In the 12 months since the partnership started we’ve seen some tremendous results,’ says Russell. ‘Working with a range of boatbuilding customers on a number of projects, we can show results that demonstrate an increase in speed of up to three or four knots compared to the manufacturer’s original expectations.

‘But for us at least, it’s certainly not just about speed. We already have one customer who wasn’t interested in going faster but wanted to improve efficiency by limiting engine RPM,’ continues Russell. ‘We heard from them recently and they have predicted that the increased efficiency they have gained will save them around £50,000 a year in running and fuel costs. That sort of saving is hard to find anywhere else.’